Super absorbent polymers (SAPs) are polymers that are capable of absorbing a large amount of water about five hundred to one thousand times the mass of their own. The SAPs are called by different names according to the developers, such as, for example, super absorbency material (SAM) or absorbent gel material (AGM). Commercial productions of SAP began for use in feminine hygiene napkins, and the use of SAP can be found in a very wide range of applications, including water combination soils for gardening, water-stop materials for engineering and construction, sheets for raising seeding, freshness-keeping agents in the food distribution field, and materials for sweating rooms, as well as personal hygiene products, such as disposable baby diapers.
The known preparation methods for SAP include an inverse suspension polymerization method or an aqueous solution polymerization method. The method using inverse suspension polymerization is disclosed in, for example, JP S-56-161408, JP S-57-158209 and JP S-57-198714. The method using aqueous solution polymerization includes several methods: a thermal polymerization method characterized by polymerization of a hydrated gel polymer during shearing and freezing, and a photo-polymerization method using UV radiations on a high-concentration aqueous solution on a conveyer belt to perform both polymerization and drying at once.
The hydrated gel polymer obtained from the polymerization reaction is subjected to drying and pulverization, and then becomes commercially available as a powder product. In order to efficiently accomplish the drying step, it is of a great importance to increase the surface area of the hydrated gel polymer as possible. To increase the surface area of the hydrated gel polymer prior to the drying process, a method can be taken into consideration that involves simply pulverizing the hydrated gel polymer prepared by thermal polymerization or photo-polymerization. In this regard, there is disclosed a process of performing a primary pulverization of a hydrated gel polymer after polymerization in order to increase the surface area of the hydrated gel polymer. But, the pulverization process inevitably renders the hydrated gel polymer sticking to the inner walls, shear blades or shear shafts of shredder equipment, which hinders the pulverization process with inefficiency.
The conventional pulverizing process, for example, has involved cutting a plate-shaped super absorbent polymer obtained as a primary product into long strips and then chopping the polymer stripes into crumbs. But, the super absorbent polymer is likely to stick to the shear blades or the shear shafts while being cut into strips, which results in inefficiency of the pulverization process. Due to the stickiness, even the nicely cut stripes of the super absorbent polymer are hard to cut into smaller crumbs with uniform size.
This precludes efficiency of the primary pulverization process of the super absorbent polymer. Such an inefficient pulverization results in a failure of the subsequent drying process and consequently a deterioration in the general properties of the final super absorbent polymer product.